ABSTRACT Infection is the leading cause of death and prolonged hospitalization in severely burned patients that survive the acute phase of injury. Loss of the skin barrier, impaired innate immunity and the prevalence of antibiotic resistant bacteria increase the susceptibility of burn patients to serious infection, leading to a great need for immunomodulation strategies aimed at priming the host response in these patients. Priming with TLR4 ago- nists have been shown to augment neutrophil antimicrobial responses and enhance resistance against infec- tion after severe burn injury in mice. However, the role of the classical MyD88- and TRIF-dependent TLR sig- naling cascades in mediating this protection, are not fully understood. Preliminary studies support a primary role for MyD88-signaling in mediating antimicrobial responses, potentially through augmentation of leukocyte metabolism via induction of the PI3K signaling pathway, but the cellular and molecular mechanisms involved are unclear. The long-term goal of this project is to better understand the distinct roles of MyD88- and TRIF- dependent signaling during immunomodulation with TLR ligands. The overall objective is to identify potential immunomodulatory strategies as a means of offering protection against infection in burn patients. The central hypothesis is that Myd88-dependent signaling is crucial in facilitating TLR agonist-induced augmenta- tion of antimicrobial responses, and that MyD88-specific TLR agonists will be highly effective in im- proving neutrophil-mediated resistance to infection after burn injury. This hypothesis will be tested by three specific aims: Aim 1: To determine whether treatment with TLR agonists that selectively activate the MyD88- and TRIF-signaling pathways can improve the host response to diverse pathogens in burned mice. This aim will be achieved by priming burned mice with MyD88-specific (CpG) or Trif-specific (PolyI:C) agonists and evaluating outcome and clearance of infection after challenge with S. aureus, C. albicans and P. aerugino- sa. Aim 2: To define the cellular mechanisms by which treatment with MyD88- and TRIF-specific agonists af- fect innate immune responses to infection after burn. Burned mice will be primed with pathway specific ago- nists as in Aim 1, and neutrophil expansion, trafficking, activation, and antimicrobial functions will be analyzed. Aim 3: To define the molecular mechanisms by which TLR agonists augment antimicrobial immunity after burn injury. In this aim, activation of the PI3K/Akt/mTOR/Hif-1? signaling pathway by TLR agonists and the role of this signaling pathway in mediating antimicrobial responses via augmentation of leukocyte metabolism will be examined. The studies proposed in this project represent an entirely novel approach to the prevention of infec- tion and sepsis which is translational and highly innovative. These studies are significant in that they will also fill important gaps in knowledge by advancing our understanding of the cellular and molecular mechanisms by which TLR-based immunomodulators improve host resistance to acute life-threatening infections.